Using Narrow Emission Lines to Test Physical Models Unifying AGNs

Abstract

We contend that quantitative measurements of nuclear narrow emission line strength can strongly constrain models that unify AGNs. The reasons for the importance of narrow-line luminosity L NLR are: a) The lines normally arise via photoionisation by the integrated UV/soft X-ray luminosity L PHOT of the central source. Thus L NLR is directly linked to a physical quantity intimately connected with the central engine but not observable from the ground. For constant covering factor we expect an approximate proportionality between L NLR and L PHOT ; this has been confirmed observationally for AGNs by estimating L PHOT from either optical non-stellar luminosity or effective ionisation parameter. b) NLRs are far enough from the photoionising source to avoid the excessive obscuration that appears able to attenuate broad-line and continuum emission. Narrow-lines are radiated isotropically unlike, eg, the radio core which may be Doppler boosted. Their variability timescale of 10 3–4 years is intermediate between those of L PHOT and any large-scale radio emission.